In the case of measuring tubes for magneto-inductive flow measuring devices, measuring tubes of plastic are known, but, for the most part, measuring tubes with a support tube of metal, especially steel, are used. In the latter case, however, there is the problem that the material of the measuring tube is conductive. In order, nevertheless, to be able to tap a voltage from the measuring electrodes, the measuring tube must be electrically insulated. For this, usually an insulating, plastic layer, a so called liner, is used. The adhesion of such materials to the metal surface is, however, not optimal. Additionally, the liner can under process conditions be mechanically deformed
Therefore, a perforated sheet is applied, which serves the liner material as support body. This support body is inserted in the support tube and supports the liner material. The German patent, DE 10 2008 054 961 A1 of the applicant discloses a number of possible embodiments of such a perforated sheet, one possibility being a sheet which is perforated only segment-wise.
A major theme is the anchoring of the liner in the support tube. One method of securement is described in the aforementioned German patent, DE 10 2008 054 961. In such case, the support body is affixed in the measuring tube by a press-fit, so that the support body is secured in the support tube only due to the accompanying compression, however, without any bonding of the materials.
This variant has basically proved itself as a practical and cost effective variant, which can be put into practice in production relatively easily. However, over longer periods of time, the compressive stress of the perforated sheet can lessen and the liner can turn in the measuring tube.
Another option of anchoring is described in German patent, DE 10 2006 018 415 A1. Involved in this case is a support body composed of two half shells, which can be manufactured in the form of perforated sheets. These are secured to the measuring tube terminally via material bonds using weld pads.
The welding of a perforated sheet has the disadvantage that, most often, only little space is left between the perforated sheet and the support tube for the cast liner material to enter. Also, temperature differences between perforated sheet and support tube can due to thermal expansion differences lead over long periods of operation to failure of the weld locations.
A grid for support of a liner is additionally shown in U.S. Pat. No. 5,773,723 A1 and US Patent 2008/0196510 A1 likewise shows in a support tube a support body embodied as a perforated sheet or grid. The function of a support is also described in these embodiments. As indicated by the figures, these support bodies have, however, no anchoring to the support tube, but, instead, are embedded in the liner without anchoring.
German Patent, DE 10 2013 11 444 284 A1 discloses, additionally, a measuring tube, in which an internal thread is present. Terminal screw thread formations on a support body engage in this internal thread. The support body can be affixed in the measuring tube through screwing, or twisting, in. Twist prevention is provided by two sockets, which engage terminally with the support body.
In the case of poly-fluorinated plastic liners, it is known that only extremely weak adhesive bonds form with the support measuring tube. An undesired twisting of the liner in the measuring tube in the case of longer use is, consequently, probable, the more so, since no additional securements of the liner are provided.
An alternative to perforated sheet is described in European Patent, EP 1 039 269 A1. In such case, a sintered material is distributed flushly in the support tube and forms a tubular support body, which extends almost completely over the inner surface of the support tube. Due to the open pored structure of the sinter material, the liner material can penetrate and anchor. The sintering offers an especially good bonding of the supporting sinter material to the steel support tube.
The sintered material is, however, comparatively expensive and additionally complex to implement.